1. Field of the Invention
The present invention relates to a method and apparatus for measuring the humidity that is present in materials produced in the form of a web. More particularly, the present invention relates to the monitoring of humidity in paper type webs during the manufacturing process.
2. Description of the Prior Art
Different types of infrared hygrometers are known in the prior art, and most of these are based on the absorption of two or more wavelengths of light or radiation in a water containing material.
The transmission meters used for the measuring of the humidity in materials produced in form of a web, in particular paper or carton, can be divided into two different groups. The first group is characterized by meters which contain so-called integrating balls, into which measuring and reference beams are applied sequentially, arranged in periods, and analyzed by one detecting instrument. In the devices of the second group, the radiation is scattered between two mirrors, into which interspace the web to be examined may be placed, creating an infrared radiation sample, the intensities of the measured and reference wavelengths of which may be simultaneously analyzed.
The aforementioned methods are connected with certain disadvantages. In those methods the intensities of the measuring and reference wavelengths passed through the material to be examined are measured against each other, and as a result the moisture content value achieved becomes very much dependent on the square unit weight of the material, the filling substances, the degree of beating of the fibers etc. factors. An additional weakness of these methods and meters is that it provides only a narrow range of measuring, because if the humidity exceeds 15 per cent, the meters are no longer dependable and give false measuring readings for a wet (moisture percentage 30-50 per cent) material. Further it is not possible to measure moisture content from materials having a high square unit weight, such as thick cartons. Often materials exceeding 200 g/m.sup.2 in their square unit weight are difficult to measure.
When applying the present methods and devices the position of the material web in the measuring interspace has an influence on the measuring result. The hygrometer and the material to be measured constitute an optical entirety, which is a total of several partial factors. The material web is generally not transparent, but it acts as a kind of a light diffuser. The light coming from the light source is not isotropic or depending on the optics applied in the middle of the light beam a greater illumination than on its edges is to be observed. In the measuring of humidity, different wavelengths are used in the water and reference channels. Because the scattering phenomenon of the light is depending on the wavelength, the disintegrating materials and the material web scatter these different wavelengths in different ways. In practice the optical signals detected by the analyzers change due to the factors mentioned above when the position of the material web is changed in the measuring interspace.